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'''Subsurface mapping by ambient noise tomography''' is the mapping underground geological structures under the assistance of seismic signals. [[Ambient noise]], which is not associated with the [[earthquake]], is the background seismic signals.<ref>{{cite web |last1=Sleeman |first1=Reinoud |title=Ambient Earth noise and instrumental noise |url=https://www.knmi.nl/kennis-en-datacentrum/achtergrond/ambient-earth-noise-and-instrumental-noise#:~:text=Ambient%20Earth%20noise%2C%20or%20background |access-date=2023-11-01}}</ref> Given that the ambient noises have low frequencies in general, the further classification of ambient noise include secondary [[microseisms]], primary microseisms, and seismic hum, based on different range of frequencies.<ref name="tonyking2">{{cite journal |last1=Nishida |first1=Kiwamu |title=Ambient seismic wave field |journal=Proceedings of the Japan Academy, Series B |date=2017-08-02 |volume=93 |issue=7 |pages=423–448 |doi=10.2183/pjab.93.026 |pmid=28769015 |pmc=5713174 |bibcode=2017PJAB...93..423N |url=https://doi.org/10.2183/pjab.93.026}}</ref> We can utilize the ambient noise data collected by [[seismometers]] (or geophones) to create images for the subsurface under the following processes. Since the ambient noise is considered as diffuse wavefield, we can correlate the filtered ambient noise data from a pair of seismic stations (or seismometers) to find the [[velocities]] of seismic wavefields.<ref>{{cite journal |last1=Benson |first1=G. D. |last2=Ritzwoller |first2=M. H. |last3=Barmin |first3=M. P. |last4=Levshin |first4=A. L. |last5=Lin |first5=F. |last6=Moschetti |first6=M. P. |last7=Shapiro |first7=N. M. |last8=Yang |first8=Y. |title=Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements |journal=Geophysical Journal International |date=2007-06-01 |volume=169 |issue=3 |pages=1239–1260 |doi=10.1111/j.1365-246X.2007.03374.x |doi-access=free |arxiv=2007.03374 |bibcode=2007GeoJI.169.1239B |s2cid=229068738 }}</ref> A 2-dimensional or 3-dimensional velocity map, showing the spatial velocity difference of the subsurface, can thus be created for observing the [[geological structure]]s. Subsurface mapping by ambient noise [[tomography]] can be applied in different fields, such as detecting the underground void space,<ref>{{cite journal |last1=Wang |first1=Yao |last2=Khorrami |first2=Mohammad |last3=Tran |first3=Khiem T. |last4=Horhota |first4=David |title=Application of ambient noise tomography for deep void detection |journal=Journal of Applied Geophysics |date=2023 |volume=209 |doi=10.1016/j.jappgeo.2022.104922 |bibcode=2023JAG...20904922W |s2cid=255338249 |url=https://doi.org/10.1016/j.jappgeo.2022.104922|url-access=subscription }}</ref> monitoring [[landslides]],<ref>{{cite journal |last1=Le Breton |first1=Mathieu |last2=Bontemps |first2=Noelie |last3=Guillemot |first3=Antoine |last4=Baillet |first4=Laurent |last5=Larose |first5=Eric |title=Landslide monitoring using seismic ambient noise correlation: challenges and applications |journal=Earth-Science Reviews |date=2021-01-28 |volume=216 |pages=1–26 |doi=10.1016/j.earscirev.2021.103518 |bibcode=2021ESRv..21603518L |s2cid=234037825 |url=https://doi.org/10.1016/j.earscirev.2021.103518}}</ref> and mapping the crustal and upper [[Mantle (geology)|mantle]] structure.<ref>{{cite journal |last1=Yao |first1=Huajian |last2=Beghein |first2=Caroline |last3=Hilst |first3=Robert D. Van Der |title=Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis - II. Crustal and upper-mantle structure |journal=Geophysical Journal International |date=2008-04-01 |volume=173 |issue=1 |pages=205–219 |doi=10.1111/j.1365-246X.2007.03696.x |bibcode=2008GeoJI.173..205Y |s2cid=29548841 |doi-access=free }}</ref>
Characteristic of ambient noise refers to several quantities that can distinguish different ambient noise, including origin, [[frequency]], property, and temporal variation.
Ambient noise, as the rising star of the seismic source for seismic research other than [[earthquake]], accounts for the naturally and anthropogenically produced seismic vibration of the background.<ref>{{cite journal |last1=Oakley |first1=David O. S. |last2=Forsythe |first2=Brandon |last3=Gu |first3=Xin |last4=Nyblade |first4=Andrew A. |last5=Brantley |first5=Susan L. |title=Seismic Ambient Noise Analyses Reveal Changing Temperature and Water Signals to 10s of Meters Depth in the Critical Zone |journal=Journal of Geophysical Research: Earth Surface |date=2021-01-13 |volume=126 |issue=2 |doi=10.1029/2020JF005823 |bibcode=2021JGRF..12605823O |s2cid=234198739 |doi-access=free }}</ref> This is different from the active seismic source created solely for seismic research or large seismic source from [[earthquake]]. [[Ocean]] is the most dominant natural origin of the ambient noise field.<ref name="tonyking4">{{cite journal |last1=Igel |first1=Jonas K. H. |last2=Bowden |first2=Daniel C. |last3=Fichtner |first3=Andreas |title=SANS: Publicly Available Daily Multi-Scale Seismic Ambient Noise Source Maps |journal=Journal of Geophysical Research: Solid Earth |date=2023 |volume=128 |issue=1 |doi=10.1029/2022JB025114 |bibcode=2023JGRB..12825114I |s2cid=255123230 |doi-access=free |hdl=20.500.11850/591152 |hdl-access=free }}</ref>
Any seismic source is transmitted as either [[Body wave (seismology)|body wave]]s or [[surface waves]], where ambient noise is no exception. Summary of their properties are shown below.
{| class="wikitable"
Data collection of ambient noise is the prior stage of subsurface mapping, which is important for further analysis and correlation. The seismic noise can be transmitted by either body wave ([[P-wave]], [[S-wave]]) or surface wave ([[Rayleigh wave]], [[Love wave]]). The seismic source can be further classified into active and passive, where ambient noise once received far less attention as other seismic source in relevant research. The usage of ambient noise field rises from 2001 when the seismologists try to retrieve the Green function from the diffused ambient noise wave field.<ref>{{cite journal |last1=Sager |first1=Korbinian |last2=Ermert |first2=Laura |last3=Boehm |first3=Christian |last4=Fichtner |first4=Andreas |title=Towards full waveform ambient noise inversion |journal=Geophysical Journal International |date=12 July 2021 |volume=212 |pages=566–590 |doi=10.1093/gji/ggx429}}</ref>▼
|+ Seismic Wave
|-
! Wave Type !! Body/Surface !! Description
|-
| [[P-wave]] || Body || 1. Can pass through solid and liquid.
2. Particle movement parallel to wave movement.
|-
| [[S-wave]] || Body || 1. Can pass through solid only.
2. Particle movement perpendicular to wave movement.
|-
| [[Rayleigh wave]] || Surface || 1. Include both longitudinal & transverse motions.
2. Amplitude decreases exponentially with increasing distance from surface.
|-
| [[Love wave]] || Surface || 1. Wave speed lower than P-wave & S-wave, but higher than Rayleigh wave.
2. Horizonal particle movement perpendicular to wave propagation.
|}
[[File:Overview Seismic Waves.jpg|thumb|Overview Seismic Waves|upright=2.5]]
The dominance of seismic wave transmission of ambient noise depends on several factors, while the research technique would determine the major type of seismic wave collected for ambient noise. For example, seismologists would often use spatial auto-correlation (SPAC) method which involve the collection and analysis of [[surface wave]].<ref>{{cite journal |last1=Nthaba |first1=Bokani |last2=Ikeda |first2=Tatsunori |last3=Nimiya |first3=Hiro |last4=Tsuji |first4=Takeshi |last5=Lio |first5=Yoshihisa |title=Ambient noise tomography for a high-resolution 3D S-wave velocity model of the Kinki Region, Southwestern Japan, using dense seismic array data |journal=Earth, Planets and Space |date=2022-06-20 |volume=74 |issue=1 |page=96 |doi=10.1186/s40623-022-01654-x |bibcode=2022EP&S...74...96N |doi-access=free }}</ref>
=== Frequency of ambient noise ===
Ambient noise is often known as [[microseism]], where ‘micro’ means very small,<ref>{{cite web |title=micro |url=https://dictionary.cambridge.org/dictionary/english/micro#google_vignette |website=Cambridge Dictionary |access-date=2023-11-08}}</ref> and ‘seism’ is an alternative name for earthquake.<ref>{{cite web |title=seism |url=https://www.merriam-webster.com/dictionary/seism |publisher=Merriam-Webster |access-date=2023-11-05}}</ref> It can be further classified based on their frequency ranges, namely hum, primary microseism and secondary microseism.<ref name="tonyking3">{{cite journal |last1=Tanimoto |first1=Toshiro |last2=Anderson |first2=Aaron |title=Seismic noise between 0.003 Hz and 1.0 Hz and its classification |journal=Progress in Earth and Planetary Science |date=2023-09-11 |volume=10 |issue=1 |page=56 |doi=10.1186/s40645-023-00587-7 |bibcode=2023PEPS...10...56T |doi-access=free }}</ref> The table below shows the comparison of frequency range between the microseisms, arranged from increasing order. Figure a also shows graph of the frequency range of microseisms.
▲=== Nature of Ambient Noise ===
From an extensive range of frequency, ambient noise can be further classified into several categories, which are based on their origins. ▼
{| class="wikitable"
==== Anthropogenic ====▼
|+ Frequency Range of [[Microseisms]]
The anthropogenic ambient noise, excluded those artificial seismic sources produced intentionally for research, are originated from human activities. Considering the ocean ambient noise source as an example, there are noises that are created unintentionally by human activities, such as shipping and offshore engineering work[ref2]. The importance of shipping reflects on the well-developed trading and commercial industries. International immigration and emigration of products and goods can be done via shipping. The shipping activities are thus becoming frequent. During the shipping process, the mechanical waves can be driven up along water surface and propagate through the ocean. Other than shipping, offshore engineering work can also produce surface waves. Engineering works, which are usually done on the continent to usually fulfil the demand of urban development, include but not limited to borehole drilling, foundation construction and geophysical surveys. Extended from the continent, reclamation has been actively carried out by many countries to create more land for urban development. Those engineering works can thus also be carried out offshore. The processes of offshore drilling and exploration create continuous mechanical waves that can also propagate through ocean. ▼
|-
! Microseisms !! Frequency Range (Hz)
|-
| Hum || 9×10<sup>−3</sup> - 8×10<sup>−2</sup>
|-
| Primary microseism || 9×10<sup>−3</sup> - 8×10<sup>−2</sup>
|-
| Secondary microseism || 3×10<sup>−2</sup> - 1
|}
=== Origin of ambient noise ===
Regarding the continental urban areas, there are more examples of human activities creating the background noise. Other than the engineering works, the urban traffic is the major component of urban ambient noise. Although the mechanical waves of the continent are not as visible than those from the ocean, they can still be transmitted via the soil and rock layers. Cars travelling on the road can produce repeatable vibration on the road which can then be transmitted through the soil layers.▼
▲
====
▲
Natural ambient noise refers to the background noise produced from the natural events. Our natural environment is not stationary but constantly changing every moment because the nature itself is continuously modified by the weather, tectonic movements and biogenic activities. They can also produce low frequency background noise that can be further analysed. Some of the most significant events are listed below. ▼
▲
▲Natural ambient noise refers to the background noise produced from the natural events.
[[Wind]] can induce weak ocean waves propagating through the ocean. The varying atmospheric [[pressure]] was hypothesized as the origin before but is inadequate to support the existence of all types of [[microseism]]s.<ref name="tonyking2" /> Instead, ocean waves are proposed as the alternative origin of natural ambient noise. For example, the ocean swells interact with the sea coast to induce hum and primary microseisms, and the interaction of sea waves with opposite direction can produce secondary microseisms.<ref name="tonyking3" />
=== Variation of ambient noise ===
To evaluate whether the collected ambient noise source can be further analysed,
== Seismic velocity structure modelling ==
▲
===
The pre-processing of ambient noise data refers to the filtering of the raw data before proceeding to further analysis (cross-correlation, inversion). Raw seismic data can be collected by either [[geophone]]s, [[seismometers]], or from authorized official bodies. There are also public ambient noise source maps available in recent years.<ref name="tonyking4" /> Since the seismometers collect all ambient seismic signals from all directions, the produced seismic waveforms may not reflect the actual background seismic vibrations. Instead, they often contain some occasional seismic signals from earthquakes and other instruments, which is unnecessary in general and thus required to be removed.<ref>{{cite journal |last1=da Silva |first1=Cicero Costa |last2=Poveda |first2=Esteban |last3=Dantas |first3=Renato Ramos da Silva |last4=Julia |first4=Jordi |title=Ambient Noise Tomography with Short-Period Stations: Case Study in the Borborema Province |journal=Pure and Applied Geophysics |date=2021-04-22 |volume=178 |issue=5 |pages=1709–1730 |doi=10.1007/s00024-021-02718-x |bibcode=2021PApGe.178.1709D |s2cid=233330462 |url=https://doi.org/10.1007/s00024-021-02718-x|url-access=subscription }}</ref>
==== Cross-correlation of ambient noise ====
[[File:Simplest case of cross correlation.jpg|thumb|Figure 1: Simplest situation of ambient noise cross correlation|upright=1.5]]
[[File:Distribution of ambient noise source amended.jpg|thumb|Figure 2: Resulting Green's functions in different distribution of ambient noise sources|upright=1.5]]
Ambient noise [[cross correlation]] is the process of finding the receiving time lag of ambient noise sources between two nearby stations. Figure 1 illustrates the simplest case of ambient noise cross-correlation. For a pair of receivers (or seismometers or seismic stations) at different locations, the ambient noise signals would be received at a different time, assumed that they travel at the same velocity at the subsurface. The products of cross-correlation of those signals are new seismic waveforms, namely [[Green's function]]. Regarding the case with multiple ambient noise sources, the shape of the cross correlation function depends on whether the ambient noise sources are evenly distributed across a certain area. For the most ideal situation where the ambient noise signals are distributed evenly across all directions, the Green’s function would be highly symmetrical (see Figure 2).
==== Inversion ====
Inversion is one of the techniques used in ambient noise tomography.
===
Before interpreting the velocity zone, it is necessary to understand how the seismic velocity varies. In general, P and S wave travel faster in high density medium. Only P wave can travel through any medium while S wave can only travel through
====
Here are some examples of the subsurface structures and features, including but not limited to the following.
▲Before interpreting the velocity zone, it is necessary to understand how the seismic velocity varies. In general, P and S wave travel faster in high density medium. Only P wave can travel through any medium while S wave can only travel through solid. Therefore, low velocity zone can refer to some vacuum space in the subsurface layer, such as void space and faults. Conversely, the high velocity zone may refer to the lithology with closely packed rocks, such as igneous rock. To correlate the velocity zone with geological structure, it is necessary to consider the size and shape of the velocity zones, and more importantly, the resolution of the subsurface velocity image. The resolution of the image can affect the scale of the subsurface we can interpret. Sometimes, fieldwork is also needed in order to better correlate the velocity map.
* Void and hole space
* [[Fault (geology)|Fault]]
* [[Fold (geology)|Fold]]
* Oil and gas
* [[Joint]]
* [[Fracture]]
* [[Groundwater]]
== See also ==
*[[Seismology]]
*[[Seismic Interferometry]]
*[[Seismic inversion|Inversion]]
*[[Green's function]]
{{Clear}}
==References==
{{Reflist}}
[[Category:Geophysical imaging]]
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